A liquid crystal display (LCD) comprises an upper substrate, a lower substrate, and a liquid crystal layer interposed therebetween. A common electrode and color filters are formed on the upper substrate, which is generally called a color filter substrate. The lower substrate is generally called an array substrate, on which a plurality of gate lines parallel to each other and a plurality of data lines parallel to each other are formed. The gate lines and data lines intersect with each other orthogonally, defining a plurality of pixel units in a matrix on the substrate. In each of the pixel unit, a pixel electrode and a switching element such as a thin film transistor (TFT) connected with the pixel electrode are included. The gate electrode of the TFT is connected with one of the gate lines, and the source electrode of the TFT is connected with one of the data lines.
The conventional method of manufacturing a TFT-LCD array substrate generally employs a five-mask (5Mask) technology or a four-mask (4Mask) technology. The 4Mask technology performs etching on both the source/drain metal layer and the active layer in the channel region of the TFT by using a gray tone mask. Such conventional 4Mask technology comprises the following steps.
Firstly, a gate metal layer is formed on the substrate by a conventional gate process, and then a gate insulating layer is formed by depositing.
Secondly, a semiconductor layer (active layer), a doped layer (ohmic contact layer), a source/drain metal layer are deposited sequentially on the substrate. A gate island of the TFT is formed by coating a photoresist layer and patterning with a gray tone mask, an ashing process is performed on the photoresist layer to expose the channel region of the TFT, the source/drain metal layer in the channel region is etched, and thereafter the doped layer and the active layer in the channel region are etched. Since it is necessary to etch the metal layer, the doped layer, and the active layer in the above processes, the photoresist in the gray toned region in the channel portion should be controlled strictly. In addition, there are strict requirements on the selection ratio and uniformity of the etching, and accordingly there are strict requirements on the process tolerance.